Although rhesus monkeys can use tools to obtain a food reward, it is unclear whether they actually understand how tools work. The purpose of this study was to determine if rhesus monkeys could readily discriminate between functional and nonfunctional tools. The monkeys were first presented with a rake-shaped tool that they could use to obtain out-of reach-treats. After monkeys demonstrated their proficiency at using the rake, they were then presented with a second task consisting of two rakes both of which were baited with out-of-reach food. However, only one rake was functional (i.e., when pulled would allow the monkey to obtain the food). The other rake was made nonfunctional by the presence of one or more conditions. In the Hole Condition, each rake rested behind a hole. For the functional rake, the hole was behind the treat whereas for the nonfunctional rake, the hole was in front of the treat thereby causing it to drop from sight as the monkey pulled the rake toward itself. In the Obstacle Condition, the board had an obstacle which either interfered with the movement of the rake when the obstacle was located in its path (nonfunctional rake) or did not interfere with the movement because the obstacle was just outside of the path of the rack (functional rake). In the Food Placement Condition, the treat was placed either on the path of the rake (functional) or just outside the path of the rake (nonfunctional). A pilot study has been conducted with one animal to refine the procedures. This subject readily learned to use the rake, but appeared to have limited understanding of its function inasmuch as the monkey retrieved treats successfully only in the Food Placement Condition. Additional subjects will be tested